Ti-6Al-4V alloy is used extensively in aerospace and aircraft because of its excellent comprehensive properties and outstanding machinability. Ti-6Al-4V alloy was welded by conventional tungsten iner-gas arc welding (TIG) and ultra-sonic frequency pulse TIG welding (U-TIG) respectively. The welding quality was tested by X-ray, and the mechanical properties of welded joint were tested , the microstructure and the fracture appearance of welded joint were observed by means of optical microscope and SEM .The results show that the joints were welded successfully by U-TIG welding, U-TIG welding could reduced pores defects of welded joint. With the increment of ultra-sonic pulse frequency, the microstructure became more and more fine and was inclined to equiaxed crystal. Moreover, the properties of joint by U-TIG welding were improved compared with that of conventional TIG. Tensile fractures of joints by TIG welding and U-TIG welding all presented character of quasi-cleavage crack, but the cellular dendrite depredation was not observed obviously in U-TIG joint. Ti-6Al-4V alloy was used extensively in aerospace and aircraft because of its excellent overall properties and outstanding machinability. Ti-6Al-4V alloy was welded by conventional tungsten iner-gas arc welding (TIG) and ultra-sonic frequency pulse TIG welding (U-TIG) respectively. The welding quality was tested by X-ray, and the mechanical properties of welded joints were tested, the microstructure and the fracture appearance of welded joints were observed by means of optical microscope and SEM. The results show that the joints were welded successfully by U-TIG welding, U-TIG welding could reduced vacuo defects of welded joint. With the increment of ultra-sonic pulse frequency, the microstructure became more and more fine and was inclined to equiaxed crystal. Tensile fractures of joints by TIG welding and U-TIG welding all presented character of quasi-cleavage crack, but the ce llular dendrite depredation was not seen obviously in U-TIG joint.